1. Field of Invention
This invention generally relates to metrology, particularly to optical metrology.
2. Related Art
Metrology techniques are used to measure distances; for example, to precisely determine surface profiles.
One metrology application is the measurement of space structures, which may be referred to as space metrology. Many systems deployed in space require highly accurate pointing and/or precise knowledge of a payload surface profile.
Some examples of such systems include current communication satellite payloads, space-based radar apparatus, and optical payloads.
Existing metrology techniques may not be satisfactory for some applications. For example, some existing space metrology techniques use cameras to monitor targets arrayed on a surface of interest. However, in order to achieve high angular accuracy, either the total number of pixels for the camera needs to be large, or the field-of-view should be small (so that the area imaged by each pixel is relatively small). In order to monitor all relevant targets with a particular accuracy, a large number of cameras may be needed. This increases the complexity of the system, while decreasing its reliability.
Some other space metrology techniques use active targets; that is, devices positioned on the surface of the structure that require power to perform one or more functions in response to received light. Some examples of active targets are photodiodes or lasers. Metrology systems using active targets may be advantageous in some circumstances, since light need not travel round-trip (as it does for passive targets), and since the system can electronically determine and/or control which target is being measured at a particular time. However, active targets increase the complexity of the system (e.g., because of the necessary cabling), and so may be more expensive and less reliable than passive targets. Active targets also increase the overall system weight, and may be difficult to package into a deployable structure.
Other existing metrology techniques use laser trackers and/or scanning lidar (light detection and ranging). These systems scan a collimated laser beam over a large field of view. The two-dimensional scan may be complex, and it may be difficult to scan the targets in a time sufficient to meet overall system metrology bandwidth requirements.
Other metrology techniques use multiple scanning fanned laser beams with active targets (such as photodiodes). The active targets generate signals in response to receiving light from the multiple scanning fanned laser beams. The signals from the targets are then processed, and the target position determined using triangulation.
Depending on the implementation, techniques using multiple scanning fanned laser beams may be impractically complex and slow, while the alignment and other intrinsic errors between the laser beams may affect the overall accuracy. Further, active targets may be less reliable and more complex than passive targets.